The present invention relates to a system and method for power augmentation, in particular for increasing the air mass flow rate into a gas turbine.
In gas turbine-electrical power generation applications, many power augmentation methods are available, but those methods are limited at the combustion reaction stage. More specifically, limits are imposed on increasing firing rate to within operating temperature limits; or on introducing a proportional amount of steam, vapor, water or other mediums into the gas path. Increasing the air mass flow rate into a gas turbine allows for an increase in fuel consumption, while maintaining a constant firing temperature. Increasing the air mass flow also allows for an increased injection flow rate of steam, water, vapor or other mediums into the gas path because a constant proportion of injection mass flow is maintained with respect to airflow. The theoretical machine limitations of increasing gas turbine inlet airflow are the power density through the gas turbine shaft arrangement, the pressure limits of the gas turbine casings, and the equipment being driven by the gas turbine.
Power augmentation with inlet pressurization is most beneficial for peaking operation at high ambient temperatures, where gas turbine shaft horsepower is significantly lower than its available capacity. Accordingly, power augmentation methods are normally applied to gas turbine generators used for peaking, load cycling and daily stop/start load demand profiles. Under these conditions, the gas turbine generator electrical output, or shaft horsepower, response is rapid during the start-up cycle and for adjusting output to reflect the load demand on the power transmission system. In general, existing facilities can only make adjustments from rated output by decreasing the inlet airflow to the gas turbine. This occurs by reducing inlet area geometry; then the gas turbine must reduce firing temperature to reflect load demand, and become less efficient as firing temperature decreases.
As an embodiment of the invention, the air mass flow rate into the gas turbine is increased with a variable speed, supercharging fan located at the gas turbine inlet, so that the gas turbine shaft horsepower can be augmented. The gas turbine""s exhaust heat is advantageously captured and converted into steam energy, which is then used to drive the variable speed, supercharging fan.
The nature of a variable speed, supercharging fan allows the gas turbine to maintain a maximum rated firing temperature, while being able to adjust the electrical output as a function of varying the supercharging fan speed. Applying variable pitch fan blades or fan propellers can further enhance the range of operation at maximum firing temperature. In comparison with existing gas turbine generator facilities, which can only make adjustments from rated output by decreasing the inlet air flow to the gas turbine, this is significant if dispatched for load cycling operation.
In addition, the rate of change of power output with a variable speed drive, supercharging fan in operation is not constrained by a thermal strain, power output control rates limiter. Again, this results from maintaining constant firing temperature and not imposing thermal transients on hot gas path parts. The gas turbine power output can change as a function of the acceleration and deceleration rate of the supercharging fan. A supercharging fan can take less than 10 seconds to accelerate or decelerate between idle speed and full speed. During an emergency start-up cycle the gas turbine generator output can dispatch with a rapid boost and power output from the supercharging fan.
Thus, the invention is embodied in an inlet pressurization system for augmenting power produced by a gas turbine system of the type including a compressor, a combustor for heating the compressed air, and a turbine for converting the energy of the hot combustion gases to work for driving the compressor and for supplying a load, the system comprising: a heat exchanger for evaporating and/or superheating a working fluid using the hot exhaust gases from the gas turbine; a supercharging fan operatively coupled to the compressor for augmenting air supply to the compressor; and an apparatus for converting energy of the superheated working fluid to shaft power for driving the supercharging fan. In one embodiment, a steam turbine drives the supercharging fan.
In an exemplary embodiment, the supercharging fan is a variable speed supercharging fan and/or the supercharging fan may have variable pitch blades.
According to a further feature of the invention, an assembly is provided to cool airflow downstream of the supercharging fan and upstream of the compressor inlet.
The invention is further embodied in a method for augmenting the power produced by a gas turbine system of the type having a compressor, a combustor, and a turbine converting the energy of the combustor hot gases to work for driving the compressor and for supplying a load, the method comprising feeding a working fluid to a heat exchanger that receives hot exhaust gases from the turbine for evaporating and/or superheating the working fluid; converting energy of said superheated working fluid into mechanical shaft horsepower; driving a supercharging fan with the mechanical shaft horsepower to generate an airflow; and directing the airflow into the compressor thereby increasing the flow of air into the inlet of the gas turbine to increase power density through the gas turbine to produce more shaft horsepower.
In an embodiment of the method of the invention, the airflow is cooled before being directing into the compressor.
In an exemplary embodiment, furthermore, the method further comprises varying fan speed to change shaft horsepower of the gas turbine while maintaining a nominal gas turbine firing temperature.